The present invention relates to a method of forming cross ribbed pipes of single and duplex metal structure, for the use in power installation heat exchangers. The invention is also directed to tools for forming a helical pipe rib configuration.
In order to form a cross rib out of pipe material, it is necessary to roll the material with three tools, each consisting of over ten disks of adequate shapes and revolving in relation to the pipe, causing it to rotate and move along its axis. This complex motion of the pipe is forced by a bevel arrangement of driven tool shafts to the pipe axis. During the rolling process, the tool performs the following forming operations: it pinches into the pipe surface, to initiate a first step of extending the rib, forming of the rib shape and; in case of duplex metal pipes, compresses the ribbed metal layer to the core pipe. The tools are provided with rolling disk sections, having the shapes and outside diameters corresponding to the particular operations of rib forming. The first three operations require disks having symmetrical profile shapes, and progressively increasing diameters and work thickness in the rolling direction. The disks performing the last operation are asymmetrical in profile and have a slightly increased diameter in order to compress the ribbed pipe metal to the core pipe. However, the internal stresses in the outside pipe and its ribs, that remain after the rolling process, cause a loosing of the tighteness between the two pipe layers which have on effect upon proper heat transfer into the ribs.
The above described process involves high thrust forces in tool bearings, created mainly at the forming and compressing disks, so that they often deflect under force impulses. This requires a sufficient disk stiffness and thickness, thus limiting the minimum pitch of the helical rib line. To have a reasonable pitch, the tool durability must be decreased, and damage to a single disk results in a production break of a hole rib forming line.